Fix ignored install_tag kwarg in install_subdir()

Fixes: #9263

mesonbuild/backend/backends.py

@@ -1700,7 +1700,7 @@ class Backend:
             if not sd.strip_directory:
                 dst_dir = os.path.join(dst_dir, os.path.basename(src_dir))
                 dst_name = os.path.join(dst_dir, os.path.basename(src_dir))
-            i = SubdirInstallData(src_dir, dst_dir, dst_name, sd.install_mode, sd.exclude, sd.subproject)
+            i = SubdirInstallData(src_dir, dst_dir, dst_name, sd.install_mode, sd.exclude, sd.subproject, sd.install_tag)
             d.install_subdirs.append(i)
 
     def get_introspection_data(self, target_id: str, target: build.Target) -> T.List['TargetIntrospectionData']:

test cases/unit/98 install all targets/custom_files/data.txt

@@ -0,0 +1 @@
+Hello World

test cases/unit/98 install all targets/meson.build

@@ -64,6 +64,10 @@ custom_target('ct2',
   install_tag: 'custom',
   install: true,
 )
+install_subdir('custom_files',
+  install_dir: get_option('datadir'),
+  install_tag: 'custom',
+)
 
 # First is custom, 2nd is devel, 3rd has no tag
 custom_target('ct3',

unittests/allplatformstests.py

@@ -3857,6 +3857,8 @@ class AllPlatformTests(BasePlatformTests):
             Path(installpath, 'usr/share/out1-custom.txt'),
             Path(installpath, 'usr/share/out2-custom.txt'),
             Path(installpath, 'usr/share/out3-custom.txt'),
+            Path(installpath, 'usr/share/custom_files'),
+            Path(installpath, 'usr/share/custom_files/data.txt'),
             Path(installpath, 'usr/lib'),
             Path(installpath, 'usr/lib/libbothcustom.a'),
             Path(installpath, 'usr/' + shared_lib_name('bothcustom')),